Data flow diagrams
A data flow diagram (DFD) shows how data moves through a system: where it comes from, where it's processed and where it's stored. It doesn't show timing or conditions; other diagrams do that.
We start with the context diagram: the whole system as one process in the middle, surrounded by everyone it deals with.
Then we open the system into smaller processes at level 1 and add data stores. (Simplified: payment is left out.)
Watch outKey rules: no direct flow between two external entities, between an entity and a data store, or between two stores; data must pass through a process. Every process needs an input and an output. Name processes with verbs (“Receive order”) and flows with nouns (“Order”).